Spectroscopy Helps Capture Charge Transfer in Solar Cells

OLDENBURG, Germany, MODENA, Italy, and MILAN, June 6, 2014 — Ultrafast spectroscopy has allowed physicists, for the first time, to observe and capture the light-to-current conversion process in an organic solar cell as it happens.

The quantum-mechanical nature of electrons and their coupling to nuclei is fundamentally important for the charge transfer in organic photovoltaic devices, according to the study, which was conducted by researchers at the University of Oldenburg, the University of Modena and the Polytechnic University of Milan.

Quantum simulation of a portion of an organic solar cell. The wavelike oscillations of an electron are shown after sunlight is absorbed. Courtesy of the Institute for Nanoscience at CNR.
Nanostructured blends of conjugated polymers are the light absorbers in organic solar cells, and fullerenes serve as electron acceptors. The light-induced transfer of an electron from polymer to fullerene is the most basic step in converting light to current, but it happens so fast that following it directly has been a challenge.

When the researchers illuminated the polymer layer in an organic solar cell using femtosecond light pulses, the pulses induced oscillatory, vibrational motion of the polymer molecules. But that wasn’t all.

"Unexpectedly … we saw that also the fullerene molecules all started to vibrate synchronously,” said lead researcher Dr. Christoph Lienau, a physics professor at the University of Oldenburg. “We could not understand this without assuming that the electronic wave packets excited by the light pulses would coherently oscillate back and forth between the polymer and the fullerene.

“In such organic blends, the interface morphology between polymer and fullerene is very complex and the two moieties are not covalently bound. Therefore, one might not expect that vibronic coherence persists even at room temperature."

So they consulted colleagues at the Institute for Nanoscience at the Italian National Research Council (CNR) and the University of Modena. A series of quantum-dynamics simulations produced movies of the electric cloud's evolution and of the atomic nuclei, which are responsible for the oscillations.

Changing an organic PV device's makeup to optimize the coupling of electrons and nuclei could help optimize efficiency, said Dr. Elisa Molinari of CNR and the University of Modena.